The present invention relates to RF receivers and in particular to an apparatus and method within a long range RF telemetry system for receiving and demodulating, using digital components, weak signals in the presence of narrow band and wide band interference.
Conventional paging systems typically consist of a central transmitter system and small portable receivers. The receivers typically do not transmit an acknowledgement that the message was received. The portable receivers are small and lightweight. However, a significant increase in size and weight would be required, using conventional technology, to enable these receivers to transmit a detectable acknowledgement back to the source of the original message.
Small, lightweight transmitters can be installed in a portable receiver such that the system remains marketable. The problem is that power is sacrificed for the reduction in size of the transmitter. Therefore, the transmission of the acknowledgement signal would only be reliable at short distances from the source of the original message or receiving station. In order for such an acknowledgement paging system to be reliable, it must be able to receive and process the weak signals over a long range in the presence of substantial interference. Conventional systems have not achieved this.
The use of direct Frequency Shift Keying (FSK) as a method of narrow band carrier modulation has been increasing steadily since the early 1980's. Its prevalence as a medium for telemetric signalling and general intelligence transmission stems in large measure from the ease with which it may be demodulated and converted to standard data protocol (i.e., RS232c). Unfortunately as FSK's use has increased, the spectrum allocated for its use has just as steadily decreased, with the end result being that co-channel interference has become a serious threat to once reliable reception. The inability of conventional discriminators and detectors to achieve capture of a signal with less than a 4:1 relative signal strength ratio is becoming a serious barrier to reliable communication, particularly from weak sources.
Devices and methods are known in the art for the demodulation of RF signals using digital technology. For example, U.S. Pat. No. 4,811,362 to Yester et al. discloses a low power digital receiver which automatically adjusts the level of the incoming RF signal, then digitizes the signal at variable sampling rates in order to minimize power consumption. The primary concern of the Yester device is to reduce the power consumption of the receiver.
The Yester device automatically adjusts the level of the incoming signal as necessary so that large signals will not be "clipped" by a saturated amplifier or A/D converter. If an analog signal is "clipped" by an amplifier, information above a certain amplitude is lost. "Clipping" by an A/D converter causes the production of a fixed output from the converter, regardless of input signal. A significant drawback with this device is that a delay exists before the incoming signal is attenuated. That is, before the system can react to a large incoming signal, part of that signal has been clipped. Therefore information has been lost.
Moreover, once the digital signal is recovered, it is processed by a D/A converter (See column 6, lines 48 to 51) followed by audio circuits. The level of interference that is tolerable in this type circuit is greater relative to a system in which the recovered signal must be processed entirely by digital circuits. As a result, the Yester device could not adequately digitally process the signal.
The method and apparatus of the present invention represent a novel technique for receiving weak RF signals in the presence of interference using digital circuitry. The prior art neither teaches nor suggest the method or apparatus according to the present invention.